## (b) To study the nature and size of the image formed by a concave mirror on a screen by using a candle and a screen (for different distances of the candle from the mirror)

Aim
To study the nature and size of the image formed by a concave mirror on a screen by using a candle and a screen (for different distances of the candle from the mirror).

Apparatus
An optical bench with three uprights, a concave mirror with holder, a burning candle, a card-board screen.

Theory

Hence as the object (burning candle) is moved from infinity towards the concave mirror, its image (position of screen) moves from mirror focus towards infinity. The two cross each other at distance 2f i.e., at the centre of curvature of the mirror.
For candle distance less than focal length, image becomes virtual and does not come on screen.

Diagram
Similar to ray diagram as shown in Experiment 1: Section B having a burning candle in place of object needle and card-board screen in place of image needle.

Procedure
Find rough focal length of the concave mirror by usual method.
Mount the concave mirror in holder in first upright and keep it near one end of the optical bench, keeping mirror face inward.
Mount the card-board screen on a second upright and keep it at distance equal to rough focal length of mirror, from first upright.
Mount the burning candle in third upright and keep it near other end of the optical
bench.
Adjust heights so that the inverted image of erect flame of burning candle is formed on screen. Move the screen to make the image sharp. The screen will be nearly at the focus of the concave mirror.
The image will be real, inverted and much more diminished.
As the burning candle is moved towards the mirror, the screen has to be moved away from it for getting a sharp flame image. The inverted image size increases.
When the position of the candle approaches centre of curvature of the mirror, the screen also approaches the same position. The image size will be equal to the actual flame size.
Now interchange the uprights. Bring candle upright nearer to mirror than the screen upright.
Move the candle further nearer. The screen has to be moved away for getting an enlarged inverted real image on screen.
As the candle reaches the focus of the mirror, the screen may not be able to get its image which will be formed at infinity i.e. beyond the length of the optical bench.

Conclusion
This change in position, nature and size of the image is according to theoretical predictions.